1. Field of the Invention
The present invention relates to a thermal printer.
2. Description of the Prior Art
The conventional thermal printers are classified into a serial recording system and a parallel recording system. In the former, the recording is achieved by a thermal head having 1.times.7 or 5.times.7 heating elements maintained in contact with a recording paper and displaced in a direction to perform the recording of a row. On the other hand in the latter system, there is employed a recording head RH having a plurality of 5.times.1 or 5.times.7 heating elements arranged in a line as shown in a perspective view in FIG. 1 wherein a recording paper RM is advanced while said recording head is maintained in a fixed position in contact with said recording paper to perform the recording of a row. The recording method with a recording head RH having a line of 5.times.1 heating elements is generally called a dot-line printing method.
The conventional recording head for such dot-line recording method has a structure as shown in FIG. 2, wherein the heating elements are arranged in a line. FIG. 3 shows the circuit for said recording head RH, wherein there are provided a common terminal CT and dot terminals DT respectively corresponding to the heating elements HE, so that the recording of a line can be achieved by supplying a current to the corresponding dot terminals. FIG. 4 shows an example of the characters recorded by such recording head RH.
In recording with the above-mentioned printer, each row of characters is composed of seven dot lines, namely from the first dot line to the seventh dot line. The recording of a row is thus completed by the printing of said seven dot lines, followed by paper advancement corresponding to three dot lines. For recording such dot line, the recording head RH can be energized in one of the following two methods. In a first method all the heating elements are simultaneously energized according to the information to be recorded. Such method requires an elevated amount of electric power and thus a large apparatus. In a second method the heating elements HE of a dot line are divided into a certain number of blocks which are energized in succession with a time delay therebetween. In such method, because of the time delay in a dot line, the heating elements of initial blocks are cooled before the completion of recording of a dot line and tend to cause the sticking phenomenon. This phenomenon is specific to thermal printers and is caused by the adhesion of recording paper to the recording head by the solidified heat-sensitive ink, causing a resistance to the paper advancement and eventually resulting in disabled advancement of paper. Consequently it is desirable to advance the paper immediately after the recording.